In recent years, as portable devices have become increasingly widespread, demand for miniature primary batteries and secondary batteries has increased. The chief types of primary batteries are manganese dry batteries or alkali manganese dry batteries or lithium batteries, and large numbers of these are used depending on the respective application. Also, as secondary batteries, considerable use has hitherto been made of nickel-cadmium accumulators, which are alkali accumulators in which an aqueous solution of alkali is employed as the electrolyte, and nickel-hydrogen rechargeable batteries, in which a hydrogen-absorption alloy is employed as the negative electrode. Recently however, lithium ion secondary batteries, which are characterized by light weight and high energy density, and employ an organic electrolyte, have suddenly appeared on the market.
Chiefly in the case of miniature secondary batteries for portable equipment, in addition to the cylindrical type and coin type, which were the typical conventional battery shapes, in recent years use of batteries of prismatic shape has increased, and, most recently, paper-form thin batteries have also appeared.
An important recent trend in the demands made on performance of such batteries is increasing demand for higher energy density of the battery. In general terms, there are two methods of indicating the energy density of a battery. One of these is volumetric energy density (Wh/l); this is used as an index of battery miniaturization. Another is weight energy density (Wh/kg); this is used as an index of battery weight reduction.
Batteries of high volumetric energy density and high weight energy density, these respectively being indices of miniaturization and weight reduction, are highly prized by the market and there is fierce competition to increase the energy density of all types of battery.
What determines the level of energy density of a battery is principally the battery active materials of the positive electrode and/or negative electrode constituting the elements for electromotive-force, but apart from these the electrolyte and separators are also important. Very vigorous efforts are currently being made to improve these elements for increasing the energy density of the battery.
Meanwhile, miniaturization and weight reduction of the battery casing, i.e., the case of the battery that accommodates these elements for electromotive-force, which previously tended to be overlooked, has been in recent years re-evaluated as an important question and positive efforts are being made to achieve improvements in this respect. If the case of the battery can be made thinner, more battery active material can be accommodated in a portion of the same shape as conventionally but of reduced thickness, enabling the volumetric energy density of the battery as a whole to be raised. Also, if the battery case can be made of lighter material of lower specific gravity, the weight of the battery as a whole can be reduced by lowering its weight for the same shape as conventionally, and the weight energy density of the battery as a whole can thereby be raised.
Adoption of the DI (Drawing and Ironing) technique for the battery case is noteworthy as a previous technique for improving volumetric energy density. Conventionally, drawing processing was chiefly employed for manufacturing battery cases using iron-based metal material, but recently the DI technique, using both drawing and ironing, has attracted attention. Known methods for manufacturing a battery case are the technique (hereinbelow called "drawing-only technique") in which a battery case of prescribed shape is manufactured by repeating a plurality of deep-drawing steps using a press, and the so-called "DI technique", which is a technique in which a cylindrical battery case of prescribed shape is manufactured from a cup-shaped intermediate product obtained by manufacturing a cup-shaped intermediate product by a deep-drawing step using a press, followed by an ironing step using an ironing machine; this technique is known from Japanese Patent Publication No. 7-99686 etc.
Compared with the "drawing-only technique", the "DI technique" has the advantages of increased productivity due to diminution in the number of process steps, weight reduction and increased capacity due to reduction in thickness of the circumferential walls of the case, and reduction in stress corrosion etc., and for these reasons its rate of utilization is increasing. Also, conventionally, in the above method of manufacture, nickel-plated steel sheet, which is of comparatively high hardness, was employed as the battery case blank material in order to ensure sufficient pressure-resisting strength of the battery case and sufficient strength of the sealing portion. This DI technique enables the thickness of the case walls to be reduced and is said to make possible an improvement in volumetric energy density of the battery of about 5%.
Also, a well known example in which the battery case is changed to a lightweight material of lower specific gravity is provided by the case of prismatic lithium batteries, in which aluminum alloy sheet (specific gravity: about 2.8 g/cc) is employed instead of the conventional rolled steel sheet (specific gravity: about 7.9 g/cc). Efforts have been made towards weight reduction of batteries for use in portable telephones and, as a result, in this case also, examples are known in which an improvement of about 10% in weight energy density of the battery as a whole has been achieved by weight reduction of the case by changing the blank material to aluminum alloy. An example of a secondary battery using such an aluminum case is disclosed in Japanese Patent Laid-Open No. 8-329908. Impact processing or drawing processing have frequently been used as methods of manufacturing battery cases using aluminum or aluminum alloy.
Although there is some variation depending on battery size, if cold-rolled steel sheet is employed, the weight ratio represented by the case to that of the overall battery weight in batteries that have been practically employed up to the present is about 10.about.20 wt. % in the case of a cylindrical nickel/hydrogen rechargeable battery or lithium ion secondary battery; in the case of a prismatic nickel/hydrogen rechargeable battery or lithium ion secondary battery, this is about 30.about.40 wt. % i.e. twice the value for the cylindrical type. Recently, by employing aluminum or aluminum alloy for the case of prismatic lithium ion secondary batteries, this value has been reduced to 20.about.30 wt. %.
While these trends to miniaturization and weight reduction of the battery case are effective in improving battery energy density, on the other hand, in batteries, chemical reactions involving changes in the substances in the charging or discharging reaction are employed, and reliability of quality and safety therefore constitute properties which are just as important in use as energy density and cannot be neglected. In the case of primary batteries that are employed exclusively for discharge, guaranteeing capacity and/or prevention of leakage over a long period of storage, and reliability of qualities such as stable discharge performance are indispensable. In the case of secondary batteries that perform repeated charging and discharging, in addition to the properties demanded for primary batteries, performance such as cycle life and safety are even more important.
Conventionally, it was extremely difficult to maintain both high energy density and quality reliability together with safety in respect of such battery cases. Specifically, if it was attempted to obtain high energy density, deformation of the battery case or cracking under abnormal conditions frequently gave rise to problems such as leakage of electrolyte. On the other hand, if the case was made strong, this often resulted in high energy density being sacrificed; an effective method of improving the trade-off relationship between these two had not been discovered.
In the techniques for manufacturing a case as indicated above, a method based on the DI technique using drawing and ironing is excellent in that it enables relative satisfaction of both improved battery energy density i.e. thin walls and light weight and battery quality reliability together with safety. However, in this connection, further improvement in performance and quality reliability together with safety has been demanded.
Demands for such battery miniaturization and weight reduction in the market for primary batteries and secondary batteries is strong and more convenience is also sought. On the other hand, quality reliability and safety of such batteries are indispensable; previously, both of these two, namely, improved battery energy density making possible battery miniaturization and weight reduction, and quality reliability and safety, were insufficiently satisfied.
Also, regarding the technique of manufacturing the case of aluminum-based metal material, with the conventional method, reduction in thickness of the case walls was insufficient and, as a result, miniaturization and weight reduction of the battery was insufficient.
The present invention was made in the light of the above problems. Its object is to provide a battery and method of manufacturing it whereby miniaturization and weight reduction of the case of cylindrical shape or prismatic shape or shape similar thereto employed in primary batteries or secondary batteries can be achieved and the energy density of the battery can be raised, and also in which battery quality reliability and safety can be satisfied.